Leaching of nickel laterite ores with sulphuric acid is a widely applied process. Some recent researches indicate that when urea is added to the leach solution in sufficient amounts, metal dissolution rates and metal recoveries into the solution markedly increase. As such, the leaching time drops to one hour and metal recoveries rise above 95%.
These improvements are due to the effect of urea on the dissolution of iron contained in the metal bearing material. When iron is attacked and forced into dissolution, other metals that are bound with iron in some kind of iron bond become easy targets for sulphuric acid and thus their dissolution rate increases markedly.
Iron removal from the leach solution is important for the recovery of other metals and for process economics. Iron is normally removed from the solution as jarosite, goethite or hematite but none of them result in a usable iron product. Jarosite and goethite produced as such are discarded as waste with no economic value. For hematite production, iron precipitation should be carried out in an autoclave at temperatures of over 200° C. and oxygen pressure of 18 bars or above. Iron can also be precipitated out of the solution as hydroxide produced by precipitation with lime to be discarded as waste.
A great deal of research has been conducted in respect of iron precipitation as hematite that can be used in industry. U.S. Pat. No. 7,294,319 describes a method for precipitating iron from zinc sulphate solution as hematite under atmospheric conditions. Similarly, WO 2007/079532 describes a hydrometallurgical method for precipitating iron in the form of hematite from leach solutions containing nickel, cobalt and iron.
The Article ‘Uniform Particles with a large surface area formed by hydrolysis of Fe2(SO4)3 with Urea’ published in 1999 in Materials Research Bulletin Vol 34 No 6 pp 905-919 describes hydrolysis of iron oxide from a solution of Fe2(SO4)3/Urea mixture under various conditions. The article indicates that slow hydrolysis of aqueous solutions of Fe2(SO4)3 with Urea in the temperature range 60-100° C. leads to a characteristic form of iron (III) hydrous oxides and basic sulfates. Thermal dehydration of this iron (III) hydrous oxides yields amorphous Fe2O3.